External-cavity semiconductor lasers, including lasers with frequency (wavelength) selective elements in the cavity, are well known for tuning of the lasers' output wavelength and have been extensively studied. For example, W. Sorin, et al., in Optics Letters 13(9), pages 731-733 (1988), describe a laser having a laser diode with one of its facets AR coated to reduce its reflectivity, a lens, a single mode optical fiber and a tunable evanescent grating reflector for providing feedback. The laser is wavelength tunable by sliding the feedback grating laterally over the fiber. P. Zorabedian et al., in Optics Letters 13(10), pages 826-828 (1988), describe another wavelength tunable laser using either a rotatable interference filter in an external Fabry-Perot cavity or an external grating reflector providing tunable frequency-selective feedback.
A problem with previously available external-cavity semiconductor lasers is their generally low output power (on the order of 10 mW cw and 200-300 mW pulsed). Further, higher output powers are associated with unstable output intensity and frequency and less than good modal quality. For many applications, such as Raman spectroscopy, industrial process control, atomic or molecular absorption spectroscopy, environmental monitoring, projection displays, nonlinear frequency conversion, and various scientific uses, it is desirable to have a compact, high power laser diode with a stable wavelength output. Monolithically integrated DBR lasers, while capable of achieving wavelength stabilization, are not easily fabricated in all laser diode materials, leaving gaps in the wavelength bands accessible to stabilization. For example, DBR lasers are not practicable at about 750-800 nm, which is an especially suitable wavelength band for Raman spectroscopy.
In U.S. Pat. No. 5,262,644, Maguire describes the use of an infrared laser source for Raman spectroscopy. For maximum utility, the laser wavelength lies between a lower limit determined by fluorescence of the material sample to be analyzed and an upper limit determined by loss in the optical fiber used to deliver light from the laser to the sample. A more fundamental upper limit may be imposed by the detector used to gather the Raman-scattered radiation. For example, the responsivity of silicon-based detectors peaks in the wavelength region 750-950 nm, and the responsivity of germanium-based detectors peaks in the wavelengtgh region 1000-1700 nm. In addition, the scattering cross-section of the Raman process varies as 1/.lambda..sup.4, making shorter wavelength excitation sources preferred, provided fluorescence does not dominate and obscure the Raman signal. Hence, the approximate wavelength band 700-1000 nm is preferred for performing Raman spectroscopy. Maguire teaches the use of a Nd:YAG laser operating at 1064 nm or a krypton ion laser, which can operate at 799 nm or 752 nm, as suitable infrared radiation sources. We have recognized that semiconductor lasers easily access the 700-1000 nm wavelength band, and that, provided the wavelength of the laser output could be sufficiently stabilized, such semiconductor lasers would also make suitable sources for Raman spectroscopy.
In U.S. Pat. No. 4,251,780 Scifres et al. describe semiconductor injection lasers that are provided with a stripe offset geometry in order to enhance and stabilize operation in the lowest order or fundamental transverse mode. In one configuration, the stripe geometry has a horn shaped or trapezoidal section connected to a straight section, in which the width of the horn shaped or trapezoidal section expands from 8 .mu.m at the straight section to 25 .mu.m at the cleaved end facet. In contrast to configurations in which the edges of the stripe waveguides are linear and orthogonal to the cleaved end facets of the lasers, the nonorthogonal angled or curved edges of the offset stripe geometries cause higher order modes to reflect or radiate out of the waveguide, thereby increasing the threshold of the higher order modes relative to the fundamental mode.
In U.S. Pat. No. 4,815,084, Scifres et al. describe semiconductor lasers and laser arrays in which lenses and other optical elements have been integrated into the semiconductor bodies of the lasers by means of refractive index changes at boundaries in the light guiding region, where the boundaries are characterized by a lateral geometric contour corresponding to surfaces of selected optical elements so as to cause changes in shape of phase fronts of lightwaves propagating across the boundaries in a manner analogous to the change produced by the optical elements. In one embodiment, a biconcave or plano-concave diverging lens element is integrated within the laser in order to counteract the self-focusing that usually occurs in broad area lasers and that can lead to optical filamentation and lateral incoherence across the laser. The diverging lens in the laser allows the laser to operate as an unstable resonator, leading to high output power and good coherence across the lateral wavefront.
Ring lasers using semiconductor gain elements have been described previously. In particular, a ring laser using broad area laser diodes or diode laser arrays in a double-pass configuration is described by Goldberg et al. in Applied Physics Letters, vol. 51, pages 871-873 (1987). Also, a single-pass ring laser using rectangular broad-area amplifiers is described by Surette et al. in IEEE Photonics Technology Letters, vol. 5, pages 919-922 (1993).
An object of the invention is to provide a high power, external cavity, semiconductor laser with a stable, single frequency (wavelength), narrow linewidth light output.
Another object of the invention is to provide a high power, wavelength stabilized, external cavity, semiconductor laser which emits a single spatial mode, diffraction-limited output beam.